Data storage tapes are often used for backup of other mass memory devices such as disk drives. In backup or restore operations, tape drives typically operate in a streaming mode, continuously recording or reading sequential data as opposed to operating in a start-stop mode with random access to data. Tapes for data storage are often formatted into multiple data tracks running linearly down the length of a tape. The drives using such tapes typically have one or more fixed data transfer rates. The tape speed typically has some acceptable variation due to motor speed and other drive mechanical tolerance. A fixed data transfer rate with variable tape speed results in a variation in the physical length of tape required to store a fixed amount of data.
FIG. 1 illustrates a simplified example using track conventions as specified by one of the Quarter-Inch Cartridge (QIC) Drive Standards. For simplicity, FIG. 1 illustrates a tape 100 with five tracks. Data starts at either end of the tape at a logical boundary (102, 104) defined by the edge of an end-of-tape marker hole (106, 108). Actual QIC tapes may have as many as 44 tracks. As illustrated in FIG. 1, track number 0 (110) is in the center of the tape. During formatting, data is first written at the logical beginning (102) of track number 0 (110) and proceeds in the direction of the arrow 112. In the QIC standard, each track has a fixed amount of data. The end of data for track 0 is illustrated by position 114. If a drive speed is relatively slow during formatting, then a relatively small amount of tape will pass over a head before one track of data is exhausted. Therefore, there will be an unused section of tape between the end of data 114 and the logical end of the track 104. In addition, even with an exact drive speed, there is a certain amount of acceptable variation in tape length which may also result in an unused section of tape. Tape between the end of data at position 114 and the end-of-track 104 is DC erased by the tape head during formatting. This is illustrated by cross-hatched areas in FIG. 1.
QIC standards (and other data tape standards) specify a serpentine track arrangement which optimizes streaming operations. As illustrated in FIG. 1, after the tape 100 has traversed the length of track number 0, the head is stepped to track number 1 and formatting proceeds in the reverse direction, starting from the logical end-of-tape 104. As illustrated, formatting then proceeds to track number 2 in the same direction as track number 0 and so forth.
As an example, consider a drive and tape compatible with the QIC-80 standard. Tapes compatible with this standard are formatted into 28 tracks. There are several alternative standard tape lengths, the longest being 1,100 feet. The specified maximum long term speed variation for the linear speed of the tape media across the recording head is +/-3.0%. A drive with a maximum specified linear speed will use essentially all of a 1,100 foot tape length to format a track. A drive with a minimum specified linear speed will format a track with 6% less active length (1,034 feet). The tape resulting from the minimum speed drive will have a 66 foot erased section of tape at one end of each track. For 28 tracks, a worst case total of 1,848 linear feet of erased tape must pass over the head. At one of the standard tape speeds of 34 inches/second, a backup using the entire tape would spend about 11 minutes of backup time in erased areas of the tape.
There is a need to improve overall data transfer times by minimizing the time spent in the erased areas of a tape. However, prior art drives have been constrained by limitations of the QIC standards. The QIC standards specify a "Ready" signal from the drive to the host indicating that the drive is ready to receive commands. This "Ready" signal is permitted only when both the tape and head are stationary. There is a need to minimize time spent in erased areas without waiting for a "Ready" signal.
This patent specification uses QIC standards as representative examples for application of the present invention. Of particular interest are QIC-80, Revision I, Sep. 2, 1992 (recording format) and QIC-117, Revision C, Jun. 2, 1992 (command set and interface). These standard documents are available from Quarter-Inch Cartridge Drive Standards, Inc., 311 East Carrillo Street, Santa Barbara, Calif. 93101.